RY THOS. STEKL. 351 



some extent overcome by the osmotic pressure of substances in the 

 soil solution, and a plant may be compelled to take up a larger 

 amount of substances like soda or magnesia than it requires, and 

 which may be decidedly harmful. Such a case is furnished in my 

 own experience by the growth of sugar-cane on soil infiltrated 

 with brackish water, on certain parts of the Clarence River, New 

 South Wales, and on what is known as Tiri land in Fiji — low-lying 

 land subject to inundation with sea-water. In such cases, quite 

 excessive amounts of chlorides are found in the juice when the 

 cane is crushed. 



Another interesting point which requires bearing in mind in 

 case of further investigation of the water from plants, is that 

 brought out by Professor H. H. Dixon*, as to the variation in 

 concentration in different parts of the plant. Dixon finds that the 

 concentration of solids is greatest in the sap from the roots, and 

 becomes less in the upper parts of the plant. This was determined 

 by the electrical conductivity and freezing-point methods in 

 samples of sap obtained by centrifuging pieces of the roots and 

 stems. The figures obtained by both methods are proportional to 

 the amount of dissolved substance present, and are in close agree- 

 ment. It was found in the case, for instance, of Acer pseudo- 

 platamus, that the sap in the branches at a height of 30 feet had 

 only two-thirds the amount of dissolved substances as that in the 

 roots. In another plant, Colocasia antiquoriim, a similar distribu- 

 tion was observed, and it is interesting to note that the droplets of 

 liquid transpired from the tips of the leaves consisted not of sap, 

 but of practically pure water. 



These results, as regards the greater concentration of the sap in 

 the roots, are certainly contrary to what I would have expected, 

 but Professor Dixon's experiments are most lucidly described, 

 bear evidence of careful execution, and seem to me quite convinc- 

 ing The Kev. W. B. Clarke,! without giving his authority, states 

 his belief that it had been ascertained that the specific gravity of 



Transpiration and ascent of Sap in Plants, Htl4, p. 11. 

 t Lor. cit., p. 187. 



